In recent years, there has been a significant increase in the production and use of biofuels, such as biodiesel and bioethanol, generally produced by transesterification using vegetable oils or animal fats and an alcohol. A byproduct of this reaction is glycerol, and there is currently a surplus of waste glycerol produced in biofuel plants.
Glycerol is a viscous liquid that is difficult to process, and it is generally disposed of by incineration or chemical processing. However, such strategies negate the positive environmental impact of using biodiesel fuels. For example, incineration of glycerol may release pollutants into the air. Rather than disposing of the glycerol, it would be better to process the glycerol into another chemically useful product.
Glycerol can be used as a feedstock in biological conversion, a process in which a microorganism enzymatically converts glycerol into specific chemical products, such as ethanol or lactic acid. Conversion of glycerol to ethanol or other products is advantageous because they have value as fuel or as feedstocks for other processes.
Anaerobic fermentation and aerobic respiration have been used for the industrial production of chemicals, and are the two methods applied to date for glycerol-based cultures. Oxygen rich respiration offers very efficient cell growth (growth rate and yield) and converts a high percentage of the carbon source into carbon dioxide and cell mass (see table below). Anaerobic fermentation, on the other hand, results in poor cell growth, but the synthesis of several fermentation products at high yields (e.g. lactate, formate, ethanol, propionate, succinate, etc.).
Respiratory vs Fermentative MetabolismAnaerobicAnaerobicAerobicVariableFermentationRespirationRespirationGrowth RateLOWIntermediateHIGHCell MassLOWIntermediateHIGHProductHIGHHigh/IntermediateLOWYieldsCapital CostLOWLOWHIGHEnergy InputLOWLOWHIGH
Producing chemicals via oxygen rich processes, however, is more costly than using anaerobic methods for two reasons. First, aerobic fermenters are more expensive to build, due to both the higher cost per unit and the need for smaller fermenters with reduced economy of scale. Secondly, the aerobic fermenters are more costly to operate than their anaerobic counterparts due to low solubility of oxygen, which in turn requires high energy input to ensure appropriate supply of oxygen to the cells. This is especially relevant for the production of commodity chemicals, where fermentation costs can represent 50-90% of the total production cost.
Therefore, anaerobic methods are usually preferred where possible, and it is typical to grow cells to a large number aerobically, and then switch the cells to anaerobic culture for the production of desired molecules. Recently, another anaerobic method has been developed to biologically ferment glycerol into ethanol and other chemicals under anaerobic conditions. This method is described in co-pending WO2007115228, incorporated herein by reference in its entirety for all purposes. While an advantage of anaerobic fermentation of glycerol is the increased percentage yield of the desired end product, a disadvantage is slower bacterial growth associated with the lack of oxygen in the system. Thus, again what is needed in the art is a better system of culturing bacteria, using glycerol as a feedstock to produce high yields.